In general, a wireless terminal compliant with IEEE 802.11a receives an RF signal on a carrier in, for example, the 5-GHz band by a wireless unit through an antenna, and then converts the received signal into a baseband signal upon execution of signal processing.
Currently, a system used in the wireless unit includes a superheterodyne system, direct conversion system, and the like.
In case of the superheterodyne system, an RF signal received by an antenna is converted to an intermediate frequency (IF), and then undergoes gain control via an IF filter. After that, the signal having the frequency of the IF band is frequency-converted into a baseband signal, and is then converted into I and Q signals by a quadrature demodulator. At this time, in the quadrature demodulator, a direct current (DC) offset is generated, and is applied to the signals. However, since the received signal has undergone the gain control in the IF band, the DC offset is fixedly applied to the gain-controlled signals. For this reason, the wireless terminal can remove the DC offset by, for example, alternating current (AC) coupling. Hence, even when the DC offset is superposed, it does not impose any serious influence.
On the other hand, in a wireless terminal which adopts the direct conversion system, an RF signal is directly converted into a baseband signal. For this reason, no IF band exists, and the baseband signal is to undergo gain control.
That is, after a DC offset generated by the quadrature demodulator is applied to the received signal, that signal undergoes the gain control. Hence, a DC offset becomes variable according to the gain control amount of the baseband signal. For this reason, the wireless terminal which adopts the direct conversion system has to cancel the DC offset according to the gain control amount. However, especially in packet communications that suffer a frequency offset, the estimation precision of the DC offset is poor, and it is difficult to subtract the DC offset from the received signal.
Many use applications demand use of the direct conversion system which requires the smaller number of components than the superheterodyne system, and a measure against the aforementioned problem caused by the DC offset in the direct conversion method has to be taken.
The IEEE 802.11a standard adopts orthogonal frequency division multiplexing (OFDM). In the OFDM, information is transmitted using a plurality of subcarriers having orthogonal relationships. Such OFDM does not use any DC subcarrier which is seriously influenced by the aforementioned DC offset or a high-pass filter (HPF). For this reason, even when a DC (neighboring) component has varied due to the DC offset or HPF, a reception performance less deteriorates.
However, when a frequency offset exists, the influence of the DC offset or HPF often reaches subcarriers (valid subcarriers) other than the DC subcarrier. For this reason, the reception performance may deteriorate considerably.